This invention relates to apparatus for cutting a metal workpiece by melting the workpiece with a melting tool such as a plasma-arc torch. Such a torch operates by ionizing a column of gas with an electric arc to produce temperatures up to 30,000 degrees F. A high velocity, high temperature plasma stream strikes the workpiece with the heat serving to melt the metal while the high velocity stream blows away the molten metal.
Apparatus which utilizes a plasma-arc torch to cut a workpiece is disclosed in Scott U.S. Pat. No. 4,338,507. In the apparatus disclosed in that patent, a collector duct is disposed beneath the workpiece in alignment with the torch. Molten metal from the workpiece falls downwardly into the duct. Liquid is sprayed into the upper end of the duct to cool the duct and to flush slag therefrom.
Cutting of the workpiece by the torch is effected as the workpiece is shifted horizontally beneath the torch. As the hot plasma stream melts through the workpiece, dross accumulates on the lower side of the workpiece adjacent the newly formed cuts. When the dross cools and hardens, it creates undesirable ridges on the underside of the workpiece. In many cases it is necessary to grind off the dross after the cutting operation has been completed.